Spheres facing to camera
Copy Rotation Constraint.
Possibly a simple way to do this is via a copy rotation constraint, to ensure the object faces the camera plane normal.

The camera looks down its -Z axis. For example sake if I put the number or some pattern on top of the default cube and make it copy the camera rotation.
(Rotate your spheres in edit mode such that the number is aligned in top view.)

Note, to reiterate the cubes are not facing the camera, rather the opposite direction to the camera.  A track to constraint will target the camera directly
Related
Make an object Always face the camera, and always be upright?
How to put rotation- and scale invariant labels to three dimensional objects (motion-tracked insert effect)?